Salt-crystal growth, also known as salt weathering, is a type of physical weathering process. Physical weathering refers to the breakdown of rocks and minerals into smaller pieces without any chemical changes.

Option 1, chemical weathering, involves the chemical alteration of rocks and minerals due to reactions with water, air, or other substances. This process does not involve the growth of salt crystals, so it is not an example of salt-crystal growth.

Option 3, biological weathering, is the process by which organisms like plants, animals, and microbes break down rocks through physical, chemical, or biological means. This process does not directly involve salt-crystal growth.

Option 4, bio-chemical weathering, is a combination of biological and chemical weathering processes. It involves the chemical alteration of rocks by organisms. Similar to option 1, this process does not include the growth of salt crystals.

Therefore, option 2, physical weathering, is the correct answer as it directly refers to the process of salt-crystal growth. In salt-crystal growth, the water in the cracks of rocks evaporates, leaving behind salt crystals that grow and exert pressure on the surrounding rock, causing it to break apart.